1. Field of the Invention
This invention relates generally to heat pump systems, and more particularly to an apparatus and a method for controlling a defrost cycle for effecting defrost of an outdoor heat exchanger coil by initiating a defrost cycle as a function of outdoor coil temperature and outdoor air temperature.
2. Prior Art
Air conditioners, refrigerators and heat pumps produce a controlled heat transfer by the evaporation in a heat exchanger of a liquid refrigerant under appropriate pressure conditions to produce desired evaporator temperatures. Liquid refrigerant removes its latent heat of vaporization from the medium being cooled and in this process is converted into a vapor at the same pressure and temperature. This vapor is then conveyed into a compressor wherein its temperature and pressure are increased. The vapor then is conducted to a separate heat exchanger serving as a condenser wherein the gaseous refrigerant absorbs its heat of condensation from a heat transfer fluid in heat exchange relation therewith and changes state from a gas to a liquid. The liquid is supplied to the evaporator after flowing through an expansion device which acts to reduce the pressure of the liquid refrigerant such that the liquid refrigerant may evaporate within the evaporator to absorb its heat of vaporization and complete the cycle.
During the heating mode, a heat pump circuit utilizes an outdoor heat exchanger serving as an evaporator wherein the evaporator may be located in ambient air at a temperature below the freezing point of water. Thus, as this cold ambient air is circulated over the heat exchanger, water vapor in the air is condensed and frozen on the surfaces of the heat exchanger. As the frost accumulates on the heat exchanger a layer of ice is built up between the portion of the heat exchanger carrying refrigerant and the air flowing thereover. This layer of ice acts as an insulating layer inhibiting the heat transfer in the coil between refrigerant and air. Additionally, the ice may serve to block narrow air flow passageways between fins utilized to enhance heat transfer. This additional effect further serves to reduce heat transfer since lesser amounts of air will be circulated in heat exchanger relation with the refrigerant carrying conduits.
To efficiently operate a heat pump in relatively low outdoor ambient air conditions it is necessary to provide apparatus for removing the accumulated frost. Many conventional methods are known such as supplying electric resistance heat, reversing the heat pump such that the evaporator becomes a condenser or other refrigerant circuiting techniques to direct hot gaseous refrigerant directly to the frosted heat exchanger.
Many of these defrost techniques utilize energy that is not effectively used for transferring heat energy to a space to be conditioned or to another end use served by the entire system. To reduce the amount of heat energy wasted or otherwise consumed in the defrost operation it is desirable to utilize a defrost system which places the refrigeration circuit in the defrost mode only when it is determined that too much frost has accumulated on the outdoor coil.
Different types of control systems have been utilized for initiating defrost. A combination of a timer and a thermostat may be used to determine when to initiate defrost. The thermostat periodically checks to see whether or not the outdoor refrigerant temperature or a temperature dependent thereon is below a selected level, and if so acts to place the system in defrost for a length of time dependent on the timer. Other types of prior art defrost initiation systems have included measuring infrared radiation emitted from the fins of the refrigerant carrying coil, measuring the air pressure differentials of the air flow flowing through the heat exchanger, measuring the temperature difference between the coil and the ambient air, utilizing an electrical device placed on the fin whose characteristics change depending on the temperature of the device, optical-electrical methods and other methods involving the monitoring of various electrical parameters.
A disadvantage of the prior defrost modes is that they are generally static systems, wherein the initiation of the defrost mode is fixed solely by the refrigerant temperature of the coil. These static systems cause efficiency degradation since defrost is not initiated at an appropriate time, and as a function of outdoor air temperature and compressor run time.
Thus, there is a clear need for a defrost system that adjusts the initiation of defrost in response to environmental conditions to optimize defrost.